In recent years, there is an increasing interest in the heat sterilization of powered instruments for use in medical and dental applications to limit the transmission of communicable diseases. Such powered instruments, over the course of their useful lives, may be subjected to many cycles of sterilization. Commercially desirable powered instruments, or handpieces, must be capable of withstanding repeated sterilization cycles without adversely affecting their operability.
Certain powered instruments, such as a dental handpiece, may have a high speed turbine rotatably supported in the proximal end of the handpiece by a pair of anti-friction bearing assemblies to accommodate radial and thrust loads applied in some dentistry procedures. These bearings are quite small, having rolling elements, or balls, about one millimeter in diameter. The raceways and balls are usually fabricated of stainless steel.
In many high speed dental handpieces, air-driven turbines rotate tools, such as drills and burrs, at velocities exceeding 400,000 rpm, and some as high as 500,000 rpm. Even without the application of sideloads, these rotational velocities cause substantial amounts of friction to develop in the bearing assembly. In virtually all commercially available dental handpieces, friction is reduced by lubricating the bearing assembly either continuously, as by flowing a lubricant mist about the assembly, or by periodic lubrication, such as during cleaning and sterilization of the handpiece.
While lubrication of the bearing assemblies ameliorates somewhat the friction problems encountered, the need for lubrication creates other problems. For instance, it is difficult to insure the delivery of the precise amount of lubricant needed. Often, this is a result of different approaches to lubrication applied by users in the field. Lubricants also create problems in sterilization.
When high speed dental handpieces were first introduced in the 1960's, the life of the typical lubricated anti-friction bearing assembly was less than about 10 hours. Over the years, some progress has been made in the manufacture of small anti-friction bearings so that, today, a commercially satisfactory dental handpiece must be capable of surviving 2,000 hours of life cycle testing. A life cycle test involves five seconds of no rotation, acceleration to design speed and running (with a five second sideload applied at nine ounces) for a combined start-run-sideload time of 10 seconds, resulting in a total cycle time of 15 seconds. The same life cycle test procedure also applies to so-called low speed handpieces, i.e. those operating in a range of 5,000-60,000 rpm.
Thorough sterilization of handpieces generally involves the application of heat, such as in an autoclave or a chemiclave. In a typical autoclave, the handpiece, including its bearing assemblies, is subjected to a steam heat environment at a temperature of about 275.degree. F. In a chemiclave, a vapor phase alcohol is used where some water and formaldehyde is also present. Typical sterilization time is about 15 to 20 minutes in both autoclave and chemiclave sterilization procedures. In a so-called dryclave procedure, temperatures may be as high as 375.degree. F.
The high temperatures applied to handpieces during the aforementioned sterilization procedures exacerbate the corrosive effects of the moisture and chemicals on the bearing assemblies, and since many of the bearing assemblies incorporate retainers usually made of phenolic with organic fibers, for spacing the balls apart, the integrity of the retainers is also adversely affected by the heat and chemical reactions.
To place in perspective the life shortening effects of heat sterilization, it is estimated that a typical high speed dental handpiece which is not subjected to sterilization would last a dentist about 20 years with a minimum of repair (10-20% replacement of parts); whereas, the same handpiece, subject to heat sterilization as described heretofore, would last 1-2 years, requiring major repairs (80-90% replacement of parts).